The present invention is generally related to lithographic duplicators and, more particularly, to a versatile system for controlling ink and moisture feed rates during printing.
The production of quality copies by lithographic means requires that the ink and moisture each be supplied at a rate proper for the demands of the lithographic master. It is also necessary that a proper balance be maintained between the ink and moisture at all times. If the amount of ink or moisture, or the balance therebetween, is not maintained within predetermined ranges, noticeable copy degradation will result. For example, excessive moisture or an excessive moisture/ink ratio will reduce the ink transferred, resulting in copies with low optical density image areas. On the other hand, low moisture or excessive ink will cause the image areas to be blurred and may result in background toning.
In general terms, one of the primary problems over the years has been that the ink and moisture requirements for producing quality copies vary significantly with changes in operating and environmental conditions. For example, variations in temperature and humidity will change the amount of moisture required by the master for quality copies. Also, certain plates or masters, both referred to herein generally as "masters" such as those of the zinc oxide type, undergo changes during copy runs which have an effect upon the amount of moisture required. The moisture/ink requirements also may be affected by the presence of additional moisture introduced into the system as new masters are loaded in sequence for relatively short copy runs, wherein each new master is "wet" and adds moisture to the system.
The present invention addresses the specific problem of master condition changes during copy runs. A large volume of modern day lithographic duplicating is done with the use of masters of the paper base type. Such masters are relatively inexpensive compared to metal plates or the like which are intended primarily for long runs or for graphic arts applications. However, one disadvantage of many paper base masters, particularly the ZnO type, is they tend to pick up moisture by absorption, or otherwise, during a course of a copy run. This often resulted in a high moisture condition which adversely affected copy quality unless the moisture feed rate was reduced to a proper level. Thus, it was necessary for an operator to monitor the copy quality and adjust the moisture feed rate during the copy run. The image areas of such masters also tend to become somewhat less oleophilic as moisture is absorbed. Therefore, it was necessary to increase the ink feed rate during the copy run. This required further operator intervention if acceptable copy quality was to be maintained.
It would be desirable to provide a control system which eliminates the necessity for operator assistance to adjust the ink and moisture feed rates when printing with paper base type masters.
Therefore, it is an object of the present invention to provide a unique ink and moisture control system for lithographic duplicators which compensates for changes in master conditions during the course of copy runs.
Another object of the present invention is to provide a novel control system which adjusts the ink and moisture feed rate during a copy run in accordance with predicted changes in master conditions due to moisture absorption or the like.
It is a further object of the present invention to provide a versatile control system with circuit means for adjusting the ink and moisture feed rates as a function of the number of copies which have been made from a particular master in order to compensate for predicted changes in master conditions during the copy run.